The invention relates to superconducting magnetic coils.
As is known in the art, the most spectacular property of a superconductor is the disappearance of its electrical resistance when it is cooled below a critical temperature T.sub.c. Another important property is the loss of superconductivity by the application of a magnetic field equal to or greater than a critical field H.sub.c. The value of H.sub.c, for a given superconductor, is a function of the temperature, given approximately by EQU H.sub.c =H.sub.o (1-T.sup.2 /T.sub.c.sup.2)
where H.sub.o, the critical field at 0.degree. K., is, in general, different for different superconductors. For applied magnetic fields less than H.sub.c, the current carrying capacity decreases monotonically with an increasing applied field.
The existence of a critical field implies the existence of a critical transport electrical current, referred to more simply as the critical current (I.sub.c) of the superconductor. The critical current is the current at which the material loses its superconducting properties and reverts back to its normally conducting state.
Superconducting materials are generally classified as either low or high temperature superconductors. High temperature superconductors (HTS), such as those made from ceramic or metallic oxides are anisotropic, meaning that they generally conduct better, relative to the crystalline structure, in one direction than another. Moreover, it has been observed that, due to this anisotropic characteristic, the critical current varies as a function of the orientation of the magnetic field with respect to the crystallographic axes of the superconducting material. Anisotropic high temperature superconductors include, but are not limited to, the family of Cu-O-based ceramic superconductors, such as members of the rare-earth-copper-oxide family (YBCO), the thallium-barium-calcium-copper-oxide family (TBCCO), the mercury-barium-calcium-copper-oxide family (HgBCCO), and the bismuth strontium calcium copper oxide family (BSCCO). These compounds may be doped with stoichiometric amounts of lead or other materials to improve properties (e.g., (Bi,Pb).sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10).
High temperature superconductors may be used to fabricate superconducting magnetic coils such as solenoids, racetrack magnets, multipole magnets, etc., in which the superconductor is wound into the shape of a coil. When the temperature of the coil is sufficiently low that the HTS conductor can exist in a superconducting state, the current carrying capacity as well as the magnitude of the magnetic field generated by the coil is significantly increased.
Referring to FIG. 1, in fabricating such superconducting magnetic coils, the superconductor may be formed in the shape of a thin tape 5 which allows the conductor to be bent around relatively small diameters. The thin tape is fabricated as a multi-filament composite superconductor including individual superconducting filaments 7 which extend substantially the length of the multi-filament composite conductor and are surrounded by a matrix-forming material 8, which is typically silver or another noble metal. Although the matrix forming material conducts electricity, it is not superconducting. Together, the superconducting filaments and the matrix-forming material form the multi-filament composite conductor. In some applications, the superconducting filaments and the matrix-forming material are encased in an insulating layer (not shown). The ratio of superconducting material to matrix-forming material is known as the "fill factor" and is generally less than 50%. When the anisotropic superconducting material is formed into a tape, the critical current is often lower when the orientation of an applied magnetic field is perpendicular to the wider surface of the tape, as opposed to when the field is parallel to this wider surface.